Aromatic dicarboxylic acids such as terephthalic acid and isophthalic acid are used to produce a variety of polyester products, important examples of which are poly(ethylene terephthalate) and its copolymers. These aromatic dicarboxylic acids may be synthesized by the catalytic oxidation of the corresponding dialkyl aromatic compound. For example, terephthalic acid (TPA) and isophthalic acid (IPA) may be produced by the liquid phase oxidation of para-xylene (p-xylene) and meta-xylene (m-xylene), respectively.
These processes typically comprise feeding one or more dialkyl aromatic hydrocarbons, fresh and/or recycled solvent or reaction medium, and catalyst components to a reactor to which a molecular oxygen-containing gas also is fed, typically near the bottom of the reactor. Conventional liquid-phase oxidation reactors are equipped with agitation means for mixing the multi-phase reaction medium. This agitation may be provided, for example, by mechanical agitation means in vessels such as, for example, continuous stirred tank reactors (CSTRs) or in bubble column reactors having relatively high height to diameter ratios. The oxygen-containing process gas rising through the liquid contents of the reactor results in agitation of the reaction mixture. Alternatively, continuous stirred tank reactors may be used, typically having a lower height to diameter ratio than bubble column reactors.
Thus, in one example of such a process, p-xylene is oxidized to produce terephthalic acid. The p-xylene may be continuously or batchwise oxidized in the primary oxidation reactor in the liquid phase, in the presence of an oxygen-containing gas such as air. In such a process, p-xylene, an oxidation catalyst composition, a molecular source of oxygen, and a solvent such as aqueous acetic acid are combined as a reaction medium in the reactor to produce a crude terephthalic acid (CTA) reaction product. Typical oxidation catalyst compositions include a cobalt compound and a manganese compound, usually in combination with a promoter such as a bromine compound. See, for example, U.S. Pat. Nos. 2,833,816, 3,089,906, and 4,314,073, the disclosures of which are incorporated herein by reference. The process conditions are highly corrosive due to the presence of acetic acid and bromine, and titanium lining equipment's are typically used. See, for example, U.S. Pat. No. 3,012,038, incorporated herein by reference. Acetaldehyde may be used as a promoter in place of bromine, in which case reactors and process equipment's made from titanium lining are not necessary. Acetaldehyde is also useful as an initiator. Because the liquid-phase oxidations of dialkyl aromatic compounds just described are highly exothermic reactions, they are commonly carried out in vented reaction vessels, the heat of reaction being removed by vaporization of the process solvent through the upper exit port.
The resulting CTA is not very soluble in the acetic acid solvent under the reaction conditions, and precipitates from the solvent to form a suspension. This crude terephthalic acid suspension includes terephthalic acid solids, a solvent acting as the suspending medium for the solids and containing a small amount of dissolved terephthalic acid; catalyst components; unreacted p-xylene; incompletely oxidized intermediate oxidation products such as para-tolualdehyde (p-TAI), para-toluic acid (p-TA), and 4-carboxybenzaldehyde (4-CBA); and organic impurities such as fluorenones that are known to cause discoloration. The crude terephthalic acid composition is discharged from the oxidation zone and subjected to any of several mother liquor exchange, separation, purification, or recovery methods, with the recovered solvent and catalyst composition being recycled directly back to the oxidation reaction or after processing, such as by catalyst recovery or solvent purification. One such purification technique is hydrogenation using a heterogeneous catalyst. These catalysts are subject to metal poisoning, reducing their activity and requiring more frequent replacement. It is desirable to minimize the amount of incompletely oxidized intermediates and the colored impurities, to reduce these subsequent purification requirements.
Other by-products of the liquid phase oxidation which are partially or completely removed from the reaction mixture in the oxidation reactor are the off-gases, which include water, solvent, unreacted oxygen and other unreacted gases found in the source of the molecular oxygen gas such as nitrogen and carbon dioxide, and additional amounts of carbon dioxide and carbon monoxide that are oxidative losses resulting in part from the catalytic decomposition of the solvent and other oxidizable compounds under the oxidation conditions. The off-gases are vented at the overhead of the oxidation reactor to a distillation column or a condenser to separate the solvent from the other off-gases such as water, carbon dioxide, carbon monoxide, nitrogen, gaseous bromine compounds such as methyl bromide, etc.
Although it is desirable to recover and recycle as much solvent as possible, the solvent is oxidatively decomposed to some extent into its constituent gaseous products, carbon dioxide and carbon monoxide, requiring a fresh source of make-up solvent. This oxidative decomposition is often referred to in the industry as solvent burn, carbon burn, or acid burn, and is generally believed to be responsible in part for the formation of carbon oxides, although a portion of the carbon oxides produced is also the result of oxidative decomposition of the dialkyl aromatics or intermediate reaction products. Controlling or reducing formation of carbon oxides significantly lowers the operating costs of the oxidation process, by allowing a greater amount of solvent to be recovered and recycled back to the oxidation zone, and possibly also by reducing yield loss from the oxidative decomposition of the aromatic reactants. However, a reduction in carbon oxides formation should not come at the expense of significantly reduced yield or conversion, or an increase in the amount of incomplete oxidation products in the crude mixture, and if possible, it would be desirable to simultaneously reduce carbon oxides formation and maintain the conversion. Typically, however, increased conversion is accompanied by an increase in carbon oxides formation.
The activity of a p-xylene oxidation using homogenous catalyst system comprising cobalt, manganese, and bromine can be increased by addition of selected species. Zirconium is known to increase the activity of a terephthalic acid catalyst system to a greater extent than an equivalent addition of cobalt. Past studies focused on the use of Group IV metal addition (such as zirconium) for greater catalyst activity, and levels of zirconium used were typically >100 ppm in the liquid phase and at temperatures as high as 180° C. or more. Further, zirconium appears to be more difficult to remove from the resulting terephthalic acid solids than the other catalyst components, that is, than cobalt, manganese and bromine. Thus, due to the higher levels of zirconium used in prior studies, significant levels of zirconium were typically detected in the resulting terephthalic acid, which had a negative impact on the subsequent hydrogenation catalyst life for processes using hydrogenation purification.
There remains a need in the art for aromatic oxidation processes using cobalt, manganese, bromine, and zirconium that minimize carbon oxides formation by allowing a relatively low reaction temperature while maintaining conversion. These and additional advantages are obtained by the present invention, as further described below.